1. Statement of the Technical Field
The invention is directed to the field of communications. In particular, the invention is directed to systems and methods for reducing bit error rates in digital chaotic spread spectrum communications systems.
2. Description of the Related Art
There are many types of communications systems known in the art, such as multiple access communications systems, low probability of intercept/low probability of detection (LPI/LPD) communications systems and spread spectrum communications systems. Many of these systems depend on constant energy symbol and/or spreading sequences. Other systems induce exploitable correlations via constant energy pulse shaping. Inherent to these constant energy pulse based waveforms, the signal energy transmitted is stationary for all practical purposes, meaning that the energy transmitted as a function of time is constant. Non-constant energy pulse spreading sequences, such as those used in coherent chaotic waveforms, have also been employed but require significantly more computational power to synchronize. However, communication signals employing non-constant energy pulse spreading sequences are typically more secure and robust to interferers.
Regardless of the type of communications system being used, one common issue in communications systems is how to minimize the bit error rate (BER). That is, what is the best method to combat errors in the recognition of individual bits in a communications stream. Bit errors are normally encountered because of the non-ideal signal transmission medium that distorts and degrades the transmitted signal; the rate of these errors decreases nonlinearly based on how the transmission channel distorts the signal and the ratio of the signal power to the noise power obtained at the receiver. In the case of digital systems, a low BER is obtained by a variety of mechanisms, including equalization, coherent combining of diverse received signals, forward error correction, interleaving, variable transmission power, and more generally transmission power margins. These measures are typically optimized when the transmitted energy per symbol is held constant. Further, there is a desire to have the capability of dynamically adjusting the symbol energy in response to a time varying environment.